1. Field of the Invention
This invention relates to a dual-mode vibrator assembly capable of generating either compressional waves or shear waves for use in seismic exploration.
2. Description of the Related Art
In vibrational seismic surveys, vibrators mounted on heavy-duty vehicles generate and inject a swept-frequency wave train into the earth. The generated wave train propagates downwardly and is reflected upwards from subterranean lithologic boundaries and is received and recorded as a composite signal having a plurality of the reflected swept-frequency signals distributed at various time-phase positions on the record. The composite record is then correlated with a replica of the original transmitted signal to produce useful seismic data.
In general, seismic signals from a physical disturbance propagate through the earth in two modes. One mode is represented by a compressional or pressure (P) wave, wherein particle motion is parallel to the wave propagation direction. The second mode is a shear (S) wave, wherein the particle motion is perpendicular to the wave propagation direction. Shear waves generated at the earth's surface may represent motion either parallel to the earth's surface or at any angle thereto. In geophysical exploration however, only the shear waves generated wherein the particle motion is parallel to the earth's surface, i.e. horizontal, are of interest. Further references to shear waves will be restricted to horizontal shear waves (S.sub.H).
Both P- and S-waves are used is seismic exploration and are injected into the earth by vibrators. In general a vibrator assembly consists of an inertia mass of several hundred to several thousand pounds. The mass includes an internal cylinder within which is mounted a reciprocable piston. At least one piston rod interconnects the piston with a baseplate in contact with the ground. An electro-hydraulic servo-valve introduces fluid under pressure alternately to opposite sides of the piston within the inertia-mass cylinder. Reciprocation of the piston and piston rod relative to the inertia mass causes the baseplate to shake the ground.
It should be understood that the axis of motion of the vibrator assembly is mounted vertically for P-wave generation. Conversely, the axis of motion of the vibrator assembly for S-wave generation is horizontal, hence the reciprocating motion is also horizontal.
Heretofore it was necessary to provide separate vibrator assemblies, one for P-wave and one for S-wave generation, as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 4,410,062 issued to Fair. Because the vibrator assemblies are very heavy, separate vehicles are usually required to handle them. Separate P- and S-wave vibrator assemblies have been mounted on a single vehicle, such as the Mertz VSP Universal Vibrator. The large size of the Mertz vehicle precludes off-road use in most cases.
A dual-mode, single-actuator vibrator assembly is disclosed in U.S. patent application Ser. No. 602,559, assigned to a company related to the assignee of this invention. The dual-mode vibrator consists of a single actuator that injects P-waves or S-waves into the earth by altering the linkage configuration connecting the vibrator assembly to the base plate.
The vibrator assembly disclosed in the aforementioned U.S. Pat. No. 4,143,736 issued to Fair consists of a single actuator mass having a plurality of parallel offset cylinder bores reciprocally retaining a like number of pistons therein. As cited in the first example, two vibrator assemblies are required to inject each wave type. For a single truck to generate both P-waves and S-waves, the P-wave vibrator assembly must be exchanged for an S-wave vibrator assembly and vice versa.